A 3D reconstruction is a data set in which individual volume elements (voxels) of a body are assigned numerical values (gray scale values). The compound view provides information about the structure of the body. 3D reconstructions are usually generated by capturing a sequence of two-dimensional images (projections) from different angles. The different angles are created by rotating—that is to say progressively changing the position of—a C-arm X-ray unit on which are mounted an X-ray source and an X-ray detector. Each projection (i.e. each image captured) is then back-projected in three dimensions. It is usually assumed in this method that the X-ray beam attenuates at an even rate along a projection beam from the X-ray source to the X-ray detector. Structures are apparent in the 3D reconstruction due to the large number of projections involved in the method.
One problem with the 3D reconstruction when working with larger bodies is that due to its limited size, the X-ray detector only captures a fan beam that does not fully cover the body under investigation. There are, in other words, some regions of the body that do not appear in the image at the X-ray detector.
DE 10 2006 041 033.5, which was published subsequent to the filing date of the present application, describes a solution to the problem of wide bodies in which two (or even more than two) projections are captured at each of the positions taken up by the C-arm X-ray unit and in which the focus detector system is rotated between projections. It is assumed here that the X-ray source remains in the same position.
The two projections are mapped onto a virtual detector by means of what is known as homography so as to produce a larger combined image that captures the entire object. This combined image is then reconstructed using a conventional reconstruction method (for example making use of what is known as the Feldkamp algorithm). A filter that extends across horizontal rows in the detector is usually employed. This filter may be a ramp filter or a Hilbert filter.
A necessary condition implicit in the generation of the virtual detector in the method filed previously is that the X-ray source does not move in a translational manner between the two projections. However effects associated with the control technology, the structure of the C-arm X-ray unit and the mobility of the latter can lead to a situation in which the source position for the two projections is no longer the same and in which the two projections actually have different source positions. This means that the projections can no longer be superimposed without the appearance of artifacts. The artifacts appear in the 3D reconstruction of the body when it is generated.